1. Field of the Invention
The present invention relates to an optical transmission equipment and an optical network and more particularly to a time division multiplexing or wavelength division multiplexing optical transmission equipment and optical network.
2. Prior Art
A conventional optical transmission equipment often has the constitution indicated below so as to transmit a wavelength division multiplexing optical signal by one optical fiber. Namely, the constitution is that when the number of wavelengths to be multiplexed is n, an optical splitting device for splitting the optical power into n parts is connected to the optical fiber, and an optical filter for passing only one wavelength is connected to the n-split output, and an optical sending or receiving device is connected to the end thereof. There is a case that a WDM device functioning as an optical splitting device and an optical filter is used. In either case, the equipment is characterized in that optical sending or receiving devices are arranged in parallel.
In an optical transmission equipment for sending or receiving signals in time division by one optical fiber using only one wavelength, a signal from the optical fiber is split into 2 parts by the optical splitting device and they are connected to the optical sending device and the optical receiving device respectively.
The aforementioned optical splitting device causes an optical power loss of at least 3 dB per split, so that the maximum launched power of the optical transmitter or the minimum received power of the optical receiver is degraded lower than the real power of the optical device. This degradation degree is particularly conspicuous when the number of splits of the optical splitting device is large, that is, the number of wavelengths to be multiplexed is large.
On the other hand, in the WDM device in which the optical splitting device and the optical filter are integrated, the optical loss is lower than that of the optical splitting device. However, the WDM device may cause an optical loss of a little less than 1 dB, so that the maximum launched power of the optical transmitter or the minimum received power of the optical receiver is also degraded lower than the real power of the optical device.
As against this, in the optical multiplexer/demultiplexer indicated in Japanese Patent Publication 6-32325, it is described that it has semiconductor light emission elements which are installed side by side on the optical axis of the optical fiber in the propagation direction of the light emitted by the emission part and the non-excited part in the number of frequencies to be multiplexed or demultiplexed, and the semiconductor optical emitting elements are arranged in the ascending order of emitted wavelengths of the emitting part, and the adjustment of the optical multiplexer/demultiplexer at the time of preparation can be simplified.
In the optical multiplexer/demultiplexer, without using a combination of the optical splitting device and the optical filter or the WDM device, light can be multiplexed and demultiplexed by use of the transparency/absorption characteristics depending on the wavelength of the semiconductor, and with respect to optical multiplexing, there is little optical loss, and with respect to optical demultiplexing, there is a defect remaining that an optical loss is caused due to light absorption in the active region while light is passing the active region. Furthermore, there is another defect that the wavelength usable for optical transmission depends on the manufacturing conditions of the semiconductor and is a fixed wavelength, so that the use of the optical multiplexer/demultiplexer is restricted to fixed two-way communication between two places.
With respect to a method of selecting and taking out one optional wavelength from a wavelength multiplexed signal, for example, a method of mechanically switching an optical filter transmitting a different wavelength and a method using temperature control described in "4.times.4 polymer thermo-optic directional coupler switch at 1.55 um" (Electronics Letters, Vol. 30, No. 8, 1994, pp 639-640) are known. However, the part volume is large, and the price is high, and switching requires a time in ms units at the fastest, so that there is a problem that the methods cannot be applied to high-speed signal switching. However, recently, as described in "Tensile Strained InGaAs/InAlAs MQW Electroabsorption Modulators" (Technical Report of the Institute of Electronic Information and Communication Engineers OQE-93-44), an art for changing the band gap by changing the applied voltage to the optical detector has been proposed.
With respect to an art for tuning an emitted wavelength, a means for changing the injection current to the emitting part and changing the refractive index which is described in "Single-Current Continuous Wavelength Tuning in a Tunable Interdigital Electrode (TIE) DBR LD" (Electro Society Convention of the Electronic Information and Communication Engineers, 1995, C-208) has been proposed.
In the aforementioned optical multiplexer/demultiplexer indicated in Japanese Patent Publication 6-32325, light can be multiplexed and demultiplexed by use of the transparency/absorption characteristics depending on the wavelength of the semiconductor. However, with respect to optical multiplexing, there is little optical loss and with respect to optical demultiplexing, there is a defect remaining that an optical loss is caused due to light absorption in the active region while light is passing the active region. Furthermore, there is another defect that the wavelength usable for optical transmission depends on the manufacturing conditions of the semiconductor and is a fixed wavelength, so that the use of the optical multiplexer/demultiplexer is restricted to fixed two-way communication between two places. To make up for this defect and execute two-way communication between many places, it is necessary to set a band gap in the semiconductor manufacturing process, so that there is a defect that since semiconductors cannot be produced in large quantities, the cost is increased.
In the aforementioned prior art, there is no special description on expansion of the number of wavelengths which is decided once in wavelength division multiplexing transmission and to expand it, it is necessary to replace the optical multiplexer/demultiplexer. However, if a user of the transmission system requests addition of the number of wavelengths in the future, it is necessary to correspond to it.